FPS stability between single and multiplayer and use memory (ram)

FPS stability between single and multiplayer and use memory (ram):

I present two situations.

1: my problème is when I play solo in minecraft I have always 120 fps stabilize and I go to multiplayer my fps varies greatly, (120-0-60-30-11-0-120 FPS).

2:my ram uses only 11% of this capacity for minecraft,minecraft dont use all the resources even if I allocate more ram.

PS:I think we should integrated the mod optifine in minecraft,because this mod is perfect for video setting management.